Three primary methods have conventionally been used for measuring the distortion imparted by a system under test or components of a system under test. The system under test is typically driven by a test signal and provides an output signal. According to one technique, the output signal may be passed through a notch filter, which may reject the fundamental frequency of the output signal. The harmonics and noise of the remaining residual signal may be measured. However, not only may the use of a notch filter remove parts of the noise in the output signal, but a separate notch filter must be constructed for every frequency that needs to be analyzed.
A second technique uses a phase-locked loop to generate a signal at the fundamental frequency of the output signal. The noise in the output signal may be evaluated based on the differences between the output signal and the signal generated by the phase-locked loop. Unfortunately, using a phase-locked loop to search for the fundamental frequency of the output signal may require a great deal of time. Furthermore, the constraints on the distortion of the phase-locked loop generated signal may be very aggressive when the output signal being examined has low distortion.
A third technique for analyzing the distortion in an output signal may involve the use of a spectrum analyzer, which may measure the individual frequency components of the output signal. For an output signal having low distortion, the spectrum analyzer itself must have very low distortion. The residual noise and distortion of the spectrum analyzer may be too great for analyzing a low distortion output signal. Additionally, measuring the individual frequency components of an output signal using a spectrum analyzer may be a slow process.